1. Field of the Invention
This invention relates generally to an imaging system, and specifically to an improved migration imaging system utilizing an imaging member having a thermoplastic imaging surface layer.
2. Description of the Prior Art
Within the art of electrophotography are imaging processes or systems which involve the migration of particles in a liquid or softenable medium to achieve an imagewise pattern. Particle migration to provide a latent image has been disclosed, for example, in processes based upon electrophoretic and photoelectrophoretic imaging of photoconductive particles dispersed in liquids. In solid mediums that are nominally not permeable, particle migration is typically facilitated by the softening of the medium by the application of heat or solvents.
Most conventional migration imaging systems will arrange the marking particles in an imagewise pattern on the softenable member before any migration is accomplished. Thus, some means must be provided for composing the particles in an image-wise pattern, and another means may be necessary to transfer the pattern to a softenable layer. Then, a further means is used to soften the layer, and another means is used to migrate the particles into the softened layer. The system is complicated and the process is time-consuming. A simpler and more efficient system is desired.
Some migration imaging systems utilize a solid migration imaging member which typically comprises a substrate, a layer of softenable material, and a layer of photosensitive marking material deposited on the softenable layer. A latent image is formed by electrically charging the member and then exposing the member to an imagewise pattern of light to discharge selected portions of the marking material layer. The entire softenable layer is then made permeable by dissolving, swelling, melting, or softening it by application of heat or a solvent, or both. Portions of the marking material that retain a differential residual charge due to the light exposure will migrate into the softened layer by electrostatic force. One example of such an imaging process is disclosed in U.S. Pat. No. 4,883,731, issued to Tam et al.
An imagewise pattern may also be composed in a solid imaging member by establishing a differential in the density of colorant particles in imaged vs. non-imaged areas. In other words, the colorant particles are uniformly dispersed and then selectively migrated such that they are further dispersed to a greater or lesser extent. The differential density determines the image. The overall quantity of particles on the substrate is unchanged. Alternatively, the particles are migrated such that certain particles agglomerate or coalesce, thus achieving a differential density.
Or, in what is known as a heat development method, a solid imaging member will include colloidal pigment particles dispersed in a heat-softenable resin film on a transparent conductive substrate. An electrostatic image is transferred to the film, which is then softened by heating. The charged colloidal particles migrate to the oppositely charged image. Image areas are thereby increased in particle density while the background areas are less dense. Heat development is described by Schaffert, R. M., in Electrophotography, (Second Edition, Focal Press, 1980) at pp. 44-47 and, in particular, in U.S. Pat. No. 3,254,997.
However, the images formed in the solid imaging members processed according to the foregoing approaches have been found to lack the image contrast, gray scale accuracy, and sharp resolution required in high-resolution image reproduction. A simpler and more efficient imaging system would be desirable.
In another imaging process known generally as adhesive transfer, a solid, multilayered donor-acceptor imaging member is used to produce image copies. The donor layer includes a uniform fracturable layer of marking particles, a marking particle release layer, and a supporting carrier or sheet. An adhesive-coated acceptor layer overlies the marking particle layer. Areas of the marking particles are softened by localized heating in an imagewise pattern such that their attraction to, or retention by, the donor portion is less than the attraction of particles to non-heated areas. The acceptor layer may then be stripped from the member, taking the imaged pattern of marking particles from the release layer.
The aforementioned adhesive-transfer systems operate on a frangible dispersion of marking particles under a separable adhesive layer. Such systems typically cannot offer high resolution image reproductions because of an inherent compromise between the frangibility of the particles in non-imaged areas vs. the cohesiveness of particles in an imaged area. For example, in a peel-away system, any imaged area of the particulate layer must be cohesive enough to be carried with the peel-away layer. However, the imaged area must break cleanly at a border with a non-imaged area. Serifs, fine lines, dot images, and the like can receive an undesirably ragged edge during such a process.
For example, International Patent Application WO 88/04237, filed Dec. 7, 1987 by Polaroid Corporation, discloses a thermal imaging medium which includes a support sheet having a surface of a heat-liquifiable material and a layer of a particulate or porous image-forming substance. A pressure-sensitive adhesive layer overlies the particulate layer. The liquifiable material is imagewise exposed to heat to cause it to flow by capillary action into the image-forming substance. With cooling, the imaged areas of the substance are thereby retained by the material on the support sheet. The adhesive layer is then peeled away, causing the unexposed areas of the particulate layer to break from the exposed areas and be carried with the adhesive layer. The support sheet retains the exposed pattern.
However, the fracturing between exposed and unexposed areas can be uneven or irregular. Moreover, the heat-softened material is expected to flow only into a certain volume of the colorant, but the flow is not restricted. The softened material can flow laterally into a volume that is adjacent the heated area and which is not part of the image to be reproduced. The perimeter of an image component (a dot, for example) would then be greater than intended. As a result, image quality can be degraded.
In general, adhesive transfer and migration imaging systems are also materials-intensive and thus are costly to operate. This is especially so in systems which consume materials that are not provided in a simple, easy-to-use, and inexpensive form.
Significant waste products are generated in many of the above-described systems. Solvent-based systems generate a solvent effluent that is hazardous, expensive to discard, and cumbersome. Adhesive transfer systems generate discarded peel-away films which are usually not reusable. Proper disposal of such waste is inconvenient and increases operating costs.
Migration imaging and adhesive transfer processes have, therefore, not been favored for image reproduction in a number of applications, especially in high-resolution or high-speed printing.